The present invention relates to cellular communication systems, more particularly to cellular communication systems that partition subcarriers into a hierarchy of prioritized layers, and even more particularly the termination of measurement-taking of higher-prioritized layers for priority-based cell reselection algorithms.
Cellular communication systems typically comprise a land-based network that provides wireless coverage to mobile terminals that can continue to receive service while moving around within the network's coverage area. The term “cellular” derives from the fact that the entire coverage area is divided up into so-called “cells”, each of which is typically served by a particular radio transceiver station (or equivalent) associated with the land-based network. Such transceiver stations are often generically referred to as “base stations”, even when particular communication standards setting bodies apply different terminology (e.g., “NodeB” in WCDMA, and “eNodeB” in LTE) for the purpose of very precisely pointing out the distinctive capabilities and architectures of their version of the base station. As the mobile device moves from one cell to another, the network hands over responsibility for serving the mobile device from the presently-serving cell to the “new” cell. In this way, the user of the mobile device experiences continuity of service without having to reestablish a connection to the network. Handovers are controlled by a system-defined cell reselection mechanism. FIG. 1 illustrates a cellular communication system providing a system coverage area 101 by means of a plurality of cells 103.
The radiofrequency spectrum that is utilized to provide mobile communication services is a limited resource that must be shared in some way among all of the users in a system. Therefore, a number of strategies have been developed to prevent one mobile device's use (both transmitting and receiving) of radio spectrum from interfering with that of another, as well as to prevent one cell's communications from interfering with those of another. Some strategies, such as Frequency Division Multiple Access (FDMA) involve allocating certain frequencies to one user to the exclusion of others. Other strategies, such as Time Division Multiple Access (TDMA) involve allowing multiple users to share one or more frequencies, with each user being granted exclusive use of the frequencies only at certain times that are unique to that user. FDMA and TDMA strategies are not mutually exclusive of one another, and many systems employ both strategies together, one example being the Global System for Mobile communication (GSM).
As designers strive to develop systems with higher and higher capabilities (e.g., higher communication speeds, resistance to interference, higher system capacity, etc.), different technical features are incorporated, including different means for sharing radiofrequency resources. To take one of a number of possible examples, the Evolved-Universal Terrestrial Radio Access Network (E-UTRAN) Long Term Evolution (LTE) technology, as defined by 3GPP TR 36.201, “Evolved Universal Terrestrial Radio Access (E-UTRA); Long Term Evolution (LTE) physical layer; General description” will be able to operate over a very wide span of operating bandwidths and also carrier frequencies. Furthermore, E-UTRAN systems will be capable of operating within a large range of distances, from microcells (i.e., cells served by low power base stations that cover a limited area, such as a shopping center or other building accessible to the public) up to macrocells having a range that extends up to 100 km. In order to handle the different radio conditions that may occur in the different applications, Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink (i.e., the communications link from the base station to the User Equipment—“UE”) because it is a radio access technology that can adapt very well to different propagation conditions. In OFDMA, the available data stream is portioned out into a number of narrowband subcarriers that are transmitted in parallel. Because each subcarrier is narrowband it only experiences flat-fading. This makes it very easy to demodulate each subcarrier at the receiver.
In the Release 8 version of the 3GPP specifications, a new concept of layers was introduced wherein a layer is an LTE/WCDMA frequency carrier or a set of GSM frequency carriers. Each layer is assigned a certain priority out of a set of hierarchical priority levels, so that some layers can be considered to have a higher (or lower) priority than others. It is further noted that a layer can be associated with one or a plurality of cells. Consequently, the layer associated with a UE's serving cell can also be associated with other, non-serving cells.
Along with the concept of layers, the Release 8 version of the 3GPP specifications also introduced a new cell reselection mechanism that is based on the priorities. This is called priority-based cell reselection. Details of layers, priorities, and corresponding algorithms that govern when to reselect to a cell belonging to a layer of a certain priority is given in the following 3GPP specifications:                GSM 45.008        WCDMA 25.331, 25.304, 25.133        LTE 36.331, 36.304, 36.133        
Apart from the reselection algorithms, these specifications also specify when a User Equipment (UE) shall measure, and also when a UE may omit measuring, a certain layer.
Although the ideas presented herein are applicable to systems conforming with any of the GSM, WCDMA, and LTE standards, for the sake of simplicity, this description will use as examples concepts and terminology associated with LTE. This should not, in any way, be construed as an implication that the various technical aspects of embodiments consistent with the invention are limited only to LTE-conforming systems, however. To the contrary, these technical aspects are applicable to any of the above-mentioned as well as other systems.
In the LTE specification (3GPP 36.304/36.133), rules are set forth that are to be used by the UE to limit those measurements that guide cell reselection. Rule decisions are based on a measured value, Srxlev (a radio quality measurement of the serving cell defined in 3GPP 36.304 and based on Reference Signal Received Power—“RSRP”), and also on a measured value, Squal (a radio quality measurement of the serving cell defined in 3GPP 36.304 based on Reference Signal Received Quality—“RSRQ”). These rules can be paraphrased as follows (the symbol “&&” represents the Boolean “AND” operation):                The UE may omit measurements of frequencies within the same layer as that which is associated with its serving cell if Srxlev>Sintrasearch_rsrp_threshold && Squal>Sintrasearch_rsrq_threshold        The UE may omit measurements of frequencies within other layers of equal or lower priority as the layer of the serving cell if Srxlev>Sintersearch_rsrp_threshold && Squal>Sintersearch_rsrq_thresholdwhere Sintrasearch_rsrp_threshold, Sintrasearch_rsrq_threshold, Sintersearch_rsrp_threshold, and Sintersearch_rsrq_threshold are each system-defined threshold values.        
When it comes to layers of higher priority than that of the current layer, the specifications call for the UE to measure such layers at least once every 60×NumberOfLayers seconds, and if the UE notes that no cell reselection is to take place, then the UE may stop making measurements on such layers. In this case, determining that no cell reselection is to take place is based on RSRP and RSRQ measurements of the higher priority layer. For each of these, at least two measurements are filtered, the measurements being spaced apart by at least a predetermined time interval. For each of the filtered RSRP and RSRQ measurements, the UE ascertains whether the filtered measurement has fallen below a predefined reselection threshold level. If this is the case for either one of them (i.e., the filtered RSRP measurement or the filtered RSRQ measurement), then no cell reselection is to take place and measurement-taking of the higher-priority layer can cease.
The inventor of the subject matter described herein has recognized that the conventional mechanism for determining when measurements of higher priority layers can cease can often result in the UE making premature decisions that no cell reselection can take place when, in fact, conditions are such that cell reselection would be appropriate. This in turn results in inefficient performance, both at the system and UE level. It is therefore desired to have improved mechanisms for determining when to cease making measurements of higher priority layers.